Police in the northern province of Quang Ninh have broken up a ring that made 516,000 fake prepaid phone cards in China and took them back to Vietnam for use as collateral for million-dollar bank loans, officers said Tuesday.

Tran Viet Cuong, Pham Van Duat, and Dao Anh Tuan have been placed under investigation for their involvement in the ring, while ring leader Nguyen Van Chung is now wanted, Quang Ninh police told a press conference.

The ring was busted after six months of investigation that followed customs officers catching a woman carrying a cardboard box containing 25,000 prepaid MobiFone cards when she attempted to pass the Mong Cai border gate on January 16.

The woman confessed to customs officers that she was hired to carry the cards to Vietnam by two unknown young men. MobiFone, one of the two leading mobile network operators in Vietnam, said it did not issue such cards and asserted that they were forged.

Investigation results show that in May 2013, Chung and Tuan came to Wenzhou City in Zhejiang Province multiple times to place an order to make bogus prepaid phone cards.

The printing business would transport the cards to Dongxing, where Chung and Tuan would hire people to get them back to Vietnam, according to police.

Chung bought VND400 million (US$18,827) worth of real MobiFone VND100,000 cards to get their numbers, and duplicated the digits onto counterfeit cards collectively worth VND51.6 billion ($2.43 million).

He then mixed the authentic and fake cards together and brought to a bank in the northern Vietnamese province of Hai Duong as VND52 billion ($2.45 million) collateral to take out a VND35 billion ($1.65 million) loan.

The bank officials were able to charge money into their mobile phone accounts after checking some of the cards. They accepted the collateral even though they could not check all of the 516,000 cards which included fakes.

The bankers were so careless that they did not follow the standard checking procedures when receiving the collateral, and thus fell victim to Chung, police commented.

A MobiFone representative told Tuoi Tre (Youth) newspaper that no such fake cards have been sold to its subscribers.

“The cards were forged only to serve their scam,” the representative said. “We have so far received no complaint about fake prepaid cards from subscribers.”